1. Field of the Invention
The invention is related to coker units and their operation, particularly in the quenching of the vapor line running from coke drums to a fractionator in a coker unit.
2. Description of Related Art
Flow rate in a coke drum vapor line is influenced by several factors including quench injection rate, quench oil properties, coke drum temperature, vapor rate and pressure drop from the coke drums to the fractionator. In prior systems, the actual rate of liquid flowing out of the vapor line into the coker main fractionator varies during the coking cycle and can go to zero liquid flow, a dry vapor line condition which can eventually lead to plugging of the vapor line. Prior systems result in either of two undesirable conditions: (1) overquench, which reduces yields and possibly reduces unit feed rates, OR (2) underquench, which leaves a vapor line without any liquid to flush the line out into the main fractionator and which will eventually shut down the coker as the vapor line cokes. Once the line cokes to the point of causing enough pressure drop from the coke drums to the main fractionator such that all the liquid evaporates, only a short time remains until the coker must be shut downxe2x80x94a very expensive event. In the prior systems, the quench cannot generally be adjusted to target its contribution to the recycle ratio. One prior method, the delta temperature control technique, could possibly target a contribution of the recycle ratio; however, the downstream temperature indicator (TI) must be located in the common part of the vapor line near the fractionator in order for this to work correctly. The problem with putting a TI in this location is that, in all likelihood, it will foul and become inaccurate. As described in the present disclosure, a TI located at the coke drum vapor line outlet into the fractionator is not accessible during operation but is easily cleaned while decoking a drum. Prior quench techniques do not consider pressure differential between the coke drum and the fractionator.
The invention is a method and apparatus for quenching the coke drum vapor line which runs from the coke drum to the main fractionator in a coker unit. The unique part of this improved quench system is that it uses both pressure differential and unit feed rates to control quench rates for a given quench oil and unit feed quality. If the composition of the coker feed or the quench oil changes significantly, a new set of quench curves should be generated to ensure proper quenching of the coke drum vapor line. The purpose of quench is to prevent the drum vapor line from plugging with carbon-based deposits. Plugging of the vapor line causes a restriction in coker unit feed rates and ultimately leads to severely limiting coker feed rates until the plug is removed. In order to remove the vapor line plug, shut down of the unit is required which results in lost coker capacity, due to the gradual slowdown and subsequent shutdown of the coker unit, and in significant economic loss. A differential pressure control technique is utilized to quench the drum vapors going to the fractionator as opposed to a temperature, delta temperature, uninsulated line or fixed flow rate control technique as used in prior systems. Vapor line quench control by differential pressure prevents over-quenching of the vapor line during a coke drum switch, unit startup, or slowdown as well as preventing under-quenching during drum warm-ups. It improves the fractionator recovery time after a drum switch and the overall liquid product yield during the drum cycle which can be reduced by over-quenching. It also prevents the vapor line from drying out at anytime, an under-quenched condition, as long as the quench oil quality and conditions do not vary significantly.